The susceptibility of predominant tomato carotenoids to thermal isomerisation during typical food preparation is reported for five varieties with distinctively different carotenoid distribution. The tomato varieties used contain distinct amounts of the following predominant carotenoids: all-trans lycopene, all-trans β-carotene, all-trans δ-carotene, all-trans lutein and the poly-cis geometrical isomer of lycopene, prolycopene. The tomatoes were subjected to thermal treatments and unit operations similar to those during food preparation: boiling, addition of vegetable cooking oil, chopping and agitation. The results indicated that, during typical cooking of tomatoes, common factors such as genotypic differences in overall carotenoid composition, the presence of oil and physical changes to tomato tissues did not result in the thermal isomerisation of all-trans lycopene, all-trans δ-carotene, all-trans γ-carotene or prolycopene. Significant amounts of all-trans β-carotene and all-trans lutein, however, were converted to the cis configurations. The presence of vegetable cooking oil did not alter the thermal stability of any carotenoids being evaluated. Examination of samples by electron microscopy indicated that heat treatment imparted changes to the physical ultrastructure of the tomato tissue, such as cell wall and organelle deformation. The observed differences in these carotenoids' relative susceptibility to thermally induced isomerisation reactions might be attributable to their differences in physical state and cellular localisation. Thus, while thermal processing reportedly alters the bioavailability of carotenoids, its effect on the geometrical isomer distribution is selective and limited. These findings are important considerations in our overall effort to gain a better understanding of carotenoid metabolism in vivo and of the physical chemistry of lycopene in vitro.
© 2001 Society of Chemical Industry